Photomasks used for projection printing, wherein the photomask is spaced from a resist coated semiconductor wafer or the like, have high resolution metallic patterns on glass substrates, with feature sizes rapidly approaching the 1 82 m-2 .mu.m range. The quality of the pattern is very critical to the semiconductor manufacturing process, thus, care is taken to fabricate such photomasks substantially free of any defects.
Maintaining the photomasks in this defect free state is essential if acceptable device yields are to be obtained. In addition to particulate contamination, the various cleaning and handling operations that the photomask is subjected to can cause a slow deterioration of the pattern. Furthermore, it has been found that electrostatic charges build up on the face of the mask resulting in arcing between portions of the metallic patterns causing deterioration thereof. When these effects are multiplied by the six or more photomasks that are required to fabricate a semiconductor device, the yield loss can become significant. For these reasons it is economically desirable to find a method whereby the mask surface can be protected from both contamination and deterioration while maintaining acceptable resolution.
One approach to protecting the photomask pattern would be to deposit a simple protective coating thereon as described in U.S. Pat. No. 3,906,133 to Flutie. That patent describes an iron oxide masking layer on a transparent substrate which has a protective nitrocellulose coating thereon of a thickness greater than the height of protrusions on the surface of a resist coated wafer to be processed. However, the Flutie patent is directed to contact printing wherein the photomask is placed in intimate contact with the photoresist coated wafer. In projection printing the photomask is spaced from the resist coated wafer and the light passing through the photomask must be focused onto the resist coating by an optical system. It has been found that such protective coatings do not have uniform thickness and cause diffraction and aberrations in the focused projection printed pattern, resulting in unacceptable product.